1. Field of the Invention
The present invention relates to a sublimation thermal transfer image recording material, and more particularly, to a sublimation thermal transfer image recording material having good half tone image reproduction ability in which a recorded color image has uniform hue regardless of its image density, as well as to an image recording material useful for multiple sublimation thermal transfer recording which can maintain good image qualities such as high image density and the good half tone images without difference of hue when the image recording material is repeatedly used multiple times of use for n-fold speed mode multiple sublimation thermal transfer recording material.
2. Discussion of the Related Art
Recently, the demand for full color recording has increased year by year. There have been known various full color recording methods including electrophotographic recording methods, ink jet recording methods and thermal transfer recording methods. Among these methods, thermal transfer recording methods are widely employed because of having the following advantages over the other recording methods:
(1) a full color image having good image qualities can be recorded relatively speedily without generating noise; and PA1 (2) operation and maintenance of the recording apparatus are relatively easy. PA1 (1) a sublimable dye is relatively expensive; PA1 (2) yellow, magenta, cyan and if necessary, black color recording materials, each individually being of equal size to the recorded image, are needed to obtain a full color image; and PA1 (3) used recording materials must be disposed of even though there may be large unused portions of the recording material. PA1 (1) a dye is excessively added to a predetermined quantity of toluene at room temperature; PA1 (2) the dye solution is stirred and settled at the temperature for about 1 day to obtain a saturated toluene solution of the dye; and PA1 (3) the absorbance of the saturated solution is measured; and (4) the solubility of the dye to toluene is determined from the obtained absorbance with a relationship between the content of the dye and absorbance (i.e., an extinction coefficient) which has previously been obtained. PA1 C.I. Disperse Yellows 1, 3, 8, 9, 16, 41, 54, 60, 77 and 116; PA1 C.I. Disperse Reds 1, 4, 6, 11, 15, 17, 55, 59, 60, 73 and 83; PA1 C.I. Disperse Blues 3, 14, 19, 26, 56, 60, 64, 72, 99 and 108; PA1 C.I. Solvent Yellows 77 and 116; PA1 C.I. Solvent Reds 23, 25 and 27; and PA1 C.I. Solvent Blues 36, 63, 83 and 105. PA1 (1) a plurality of dye groups having different hues are dispersed in a binder resin solution so that each of values Dn is greater than about 0.5.times.Dh to form an ink layer coating liquid (A), or a plurality of dye groups having different hues are dispersed in a resin solution so that each of E values is greater than about 0.5.times.Eh to form an ink layer coating liquid (B), and then the ink layer coating liquid (A) (or (B)) is coated on a substrate and then dried to form an ink layer. The ink layer preferably has two or more layers which are overlaid. The ink layers may be aged after being dried, if required. PA1 (2) an ink layer coating liquid (A) (or (B)) in which dyes are dispersed in a particulate state in a binder resin so that each of values Dn (or En) is more than 0.5.times.Dh (or more than 0.5.times.Eh) is coated on a substrate and dried to form a lower ink layer (hereinafter referred to as a dye-supplying layer), and then another ink layer coating liquid (A)' (or (B)') in which at least one dye is dispersed in a molecular state is coated on the dye-supplying layer to form an upper ink layer (hereinafter referred to as a dye-transferring layer). The dye-transferring layer may include a resin layer having relatively low dye receivability. The ink layers may be aged after being dried, if required. PA1 (3) an ink layer coating liquid (A) (or (B)) in which dyes are dispersed in a particulate state in a binder resin so that each of values Dn (or En) is more than 0.5.times.Dh (or more than 0.5.times.Eh) is coated on a substrate and dried to form a dye-supplying layer, and then a resin layer coating liquid which includes a resin having relatively low dye receivability and a solvent is coated thereon to form a resin layer having relatively low dye receivability. The ink layers may be aged after being dried. PA1 (4) an ink layer coating liquid (A) (or (B)) in which dyes are dispersed in a particulate state in a binder resin so that each of values Dn (or En) is more than 0.5.times.Dh (or more than 0.5.times.Eh) is coated on a substrate and dried to form a dye-supplying layer, and then an ink layer coating liquid (A)' (or (B)') in which at least one dye is dispersed in a molecular state is coated on the dye-supplying layer to form a dye-transferring layer, and further a resin layer coating liquid which includes a resin having relatively low dye receivability and a solvent is coated thereon to form a resin layer having relatively low dye receivability. The ink layers may be aged after being dried. PA1 (1) selecting dyes which have different hues and which can produce a color image such as one of the three primary colors (i.e., yellow, magenta and cyan color) or one of special colors (e.g., flesh color or the like) so that the resultant recording material has good dye transferability and a recorded image has good image density, good reproducibility of the color, and light resistance; and PA1 (2) determining a content of each dye so that each of values Dn is greater than about 0.5.times.Dh, and if there is a value Dn which is less than 0.5.times.Dh, a dye which has relatively large solubility to toluene and which has the same hue as the dye groups whose value Dn is less than 0.5.times.Dh is added to increase he value Dn to be greater than 0.5.times.Dh. PA1 (1) a dye-transferring coating liquid which includes a resin, the same dye groups as used in the dye-supplying layer, the ratio of the content of each dye group being almost equal to that of the dye-supplying layer, and a solvent to which the dyes included in the dye-supplying layer have almost equal solubility, is coated on the dye-supplying layer to maintain the ratio of the content of each dye group in the dye-transferring layer to be substantially the same as that of the dye-supplying layer even when the dyes in the dye supplying layer are migrated to the dye transferring layer when the dye transferring layer coating liquid is coated. PA1 (2) a dye-transferring coating liquid which includes a resin, the same dye groups as used in the dye-supplying layer, the ratio of the content of each dye group being substantially the same as that of the dye-supplying layer, and a solvent to which the dyes included in the dye-supplying layer have relatively small solubility (less than about 20 g/l), is coated on the dye-supplying layer to prevent the dyes in the dye-supplying layer from migrating to the dye-transferring layer or to minimize the migration, resulting in maintenance of the ratio of the content of each dye group in the dye-supplying layer and the dye-transferring layer. PA1 alcohol solvents such as methyl alcohol, ethyl alcohol, allyl alcohol, propyl alcohol, butyl alcohol, amyl alcohol, 3-methoxybutyl alcohol, hexyl alcohol, 2-methyl pentanol, sec-hexyl alcohol, 2-ethyl butyl alcohol, heptyl alcohol, sec-heptyl alcohol, octyl alcohol, 2-ethylhexyl alcohol, sec-octyl alcohol, nonyl alcohol, 2, 6-dimethyl-4-heptanol, trimethylnonyl alcohol and the like; and glycol ether solvents having a hydroxy group such as ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monoisopropyl ether, ethylene glycol monobutyl ether, ethylene glycol monoisoamyl ether, ethylene glycol monohexyl ether, ethylene glycol mono-2-ethylbutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, propylene glycol monoethyl ether and the like. PA1 (3) when a dye-transferring layer coating liquid which includes a resin, the same dye groups as used in the dye-supplying layer, and a solvent to which the dyes included in the dye-supplying layer have different solubility, is coated on the dye-supplying layer, a dye which is included in the dye supplying layer and whose solubility to the solvent included in the dye-transferring layer coating liquid is relatively small compared to the other dyes included in the dye supplying layer should be contained in the dye-transferring layer coating liquid in an amount of content greater than the other dyes to maintain the ratio of the content of each dye group in the dye-transferring layer to be substantially the same as that of the dye-supplying layer in consideration that the extent to which the dyes in the dye-supplying layer are migrated to the dye-transferring layer corresponds to their solubility in the dye-transferring layer solvent. PA1 (4) when a dye-transferring layer coating liquid including a solvent to which the dyes included in the dye-supplying layer have different solubility, is coated on the dye-supplying layer, another method which can be used to make the ratio of the content of each dye group substantially the same as that of the dye-supplying layer is to add, in the dye-transferring layer coating liquid, an additional dye whose solubility to the solvent included in the dye-transferring layer coating liquid is relatively large compared to the other dyes included in the dye-supplying layer and whose hue is the same as the dye group which has a relatively small solubility to the solvent of the dye-transferring layer coating liquid. PA1 (1) a recording material in which a dye-transferring layer including at least one dye or a resin layer including a resin having a low dye receivability and at least one dye is formed on a dye-supplying layer which includes dyes and is formed on a substrate; PA1 (2) a recording material in which a dye-transferring layer including no dye or a resin layer including a resin having a low dye receivability and no dye is formed on a dye-supplying layer which includes dyes and is formed on a substrate; or PA1 (3) a recording material in which a dye-transferring layer including at least one dye is formed on a dye-supplying layer which includes dyes and which is formed on a substrate and further thereon a resin layer including a resin having a relatively low dye receivability and no dye is formed, the lower an ink layer is located, the higher dye content and/or the larger dye diffusion coefficient the ink layer preferably has. PA1 (1) an n-time mode multiple recording method in which an image is formed on a receiving material using the above-mentioned one-time recording method but the recording material is repeatedly used n-times; and PA1 (2) an n-fold speed mode multiple recording method in which an image is formed on a receiving material while the recording material is fed at a speed of 1/n that of the receiving material. PA1 (1) both of a dye-supplying layer coating liquid and a dye-transferring layer coating liquid are coated on a respective substrate made of the same material and dried to form two sheets of single-ink-layer type recording materials so that each coating weight of the coated layers is substantially the same; PA1 (2) each of the prepared recording materials is superimposed on a respective sheet of the same receiving material so that the coated surface of each recording material contacts the receiving layer of the receiving material, and heat is applied from the back side of each recording material, namely, heat is applied from the side of the substrate opposed to the ink layer, to record an image on the receiving layer; and PA1 (3) image density of each recorded image is measured. PA1 (1) the dye concentration in the dye supplying layer is higher than that in the dye transferring layer; and/or PA1 (2) the diffusion coefficient of the dye supplying layer is greater than that of the dye transferring layer. PA1 (1) preparing a coating liquid by mixing a resin solution having a solid content of from 5 to 20% by weight and a silicone oil which is a mixture of SF8411 and SF8427 (both of which are manufactured by Toray Silicone Industries Inc.) mixed in a ratio of 1/1 so that the ratio of the silicone oil to the solid of the resin is 0.3/1; PA1 (2) coating the coating liquid on a sheet of synthetic paper, Yupo FPG#95 manufactured by Oji Yuka Synthetic Paper Co., Ltd., and drying the coated liquid for 1 minute at 70.degree. C. to form a receiving layer so that the thickness of the receiving layer is 10 .mu.m on a dry basis; PA1 (3) aging the thus obtained receiving material at room temperature for more than 1 day; PA1 (4) superimposing a cyan colored recording material, e.g., a cyan colored recording material used for Mitsubishi Color Video Copy Processor SCT-CP200, on the receiving layer of the receiving material and recording an image on the receiving layer by imagewise heating the back side of the recording material using a thermal printhead, e.g., KMT-85-6MPD4 (manufactured by Kyocera Corp.), having a dot density of 6 dots/mm and an average electric resistance of 542 .OMEGA., under a condition of applied energy of 2.00 mJ/dot; and PA1 (5) measuring the image density of the recorded image with a Macbeth reflection densitometer RD-918. PA1 (1) forming a semiconductive layer on a substrate which includes a heat resistant resin such as polyester, polycarbonate, triacetyl cellulose, nylon, polyimide and aromatic polyamide, and powder of a metal such as aluminum, copper, iron, tin, nickel, molybdenum and silver which is dispersed in the heat resistant resin, and then forming an ink layer including a sublimable dye on the semiconductive layer; or PA1 (2) forming a semiconductive layer including powder of the above-mentioned metal described in method (1) on a substrate by an evaporation or a sputtering method and then forming an ink layer including a sublimable dye on the semiconductive layer. PA1 (1) measuring the coating weight of the receiving layer when the receiving layer is formed; PA1 (2) cutting a sheet of the receiving material 50 mm wide and 100 mm long, and measuring the weight of the sheet; PA1 (3) dipping the sheet into 500 g of methyl ethyl ketone (or a good solvent for the binder resin in the receiving layer) for ten minutes; PA1 (4) pulling up the sheet from the methyl ethyl ketone and measuring the weight of the sheet after drying the solvent included in the sheet; and PA1 (5) obtaining the degree of gelation by the following equation: EQU (degree of gelation)={1-(weight difference between the sheet before dipping and after dipping)/(coating weight of the receiving layer of 50 mm wide and 100 mm long)}.times.100(%).
The thermal transfer recording methods are broadly classified into a thermofusing thermal transfer recording method which transfers thermofusible ink onto a receiving material to form an image thereon and a sublimation thermal transfer recording method which transfers a thermo-diffusional dye (hereinafter referred to as a sublimable dye) onto a receiving material to form an image thereon. The sublimation thermal transfer recording method is superior to the thermofusing thermal transfer recording method because of having excellent half tone images caused by the transfer of a molecule of the thermo-diffusional dye. Therefore the sublimation thermal transfer recording method is a suitable method for full color recording.
In sublimation thermal transfer recording, a sublimation dye image can be obtained on a sublimation thermal transfer image receiving material (referred to as a receiving material) upon application of heat with a thermal printhead, laser or the like to the back side of a sublimation thermal transfer image recording material (referred to as a recording material) having an ink layer which contacts the receiving material and which includes a sublimable dye. The recording material includes a substrate and an ink layer which is formed on the substrate and includes a sublimable dye dispersed in a binder resin. A full color image can typically be obtained by appropriately transferring a yellow color dye, a magenta color dye, a cyan color dye and, if necessary, a black color dye onto a receiving material. The recording material may include a heat resistant layer on the back side thereof to make the recording material resistant to heat applied with thermal printheads. The receiving material includes a substrate and optionally an image receiving layer (referred to as a receiving layer) which is formed on the substrate. When heat is applied to the recording material, the sublimable dye diffuses into the receiving material or the receiving layer of the receiving material, so that an image is formed on the receiving material.
Sublimable dyes for use in the recording material have to have good diffusing ability under a heating condition in which a thermal printhead at high temperature (hundreds of degrees centigrade) contacts the recording material for a moment (several milliseconds) and have to have good color tone and good light resistance, to form an image having good image qualities such as high image density, good color tone reproducibility and good light resistance of the recorded image. In addition, the sublimable dyes have to be safe. There are few sublimable dyes having all of these properties. Therefore, a plurality of yellow dyes, magenta dyes, cyan dyes and if necessary, black dyes are indeed used for forming a full color image, although it is preferable that a full color image can simply be formed with one kind each of a yellow dye, a magenta dye and a cyan dye. For example, a magenta color recording material generally includes a red dye and a violet dye, a cyan color recording material includes a blue dye and a green dye and a black color recording material includes yellow, magenta and cyan dyes.
Recording materials are typically manufactured with a gravure coating method. When an image is recorded using a recording material which has a one layer type ink layer coated by a gravure coating method, the image tends to be uneven because the coated ink layer has unevenness corresponding to the form of cups of the gravure plate. Therefore, an ink layer is generally formed by coating twice a recording layer coating liquid including a resin and a sublimable dye which are dissolved or dispersed in a solvent (Japanese Laid-Open Patent Publication No. 63-302089) to form an even ink layer. Even in this case, when half tone images are recorded using a recording material having a recording layer including two or more sublimable dyes having different hues, the recorded half tone image tends to have different color tone depending on its image density. For example, when half tone images are recorded using a magenta colored recording material including a red sublimable dye and a violet sublimable dye, the recorded half tone image having relatively low image density has relatively violet-like (or reddish) magenta color compared to the half tone image having relatively high image density.
In addition, the sublimation thermal transfer recording method costs more than other methods, because:
To obviate these shortcomings, so-called multiple sublimation thermal transfer recording methods and recording materials therefor have been proposed. The multiple sublimation thermal transfer recording methods include an n-time (n is at least 2) mode multiple sublimation thermal transfer recording method and an n-fold (n is more than 1 and generally 5 to 20) speed mode multiple sublimation thermal transfer recording method. A recording material for the multiple sublimation thermal transfer recording methods is disclosed which can produce images having good image qualities such as high image density even in a large-n-time or a large n-fold multiple sublimation thermal transfer recording method. The recording material has two or more overlaid ink layers which are, for example, a dye-supplying layer which is formed on a substrate and which includes a sublimable dye dispersed in a resin, and a dye-transferring layer formed on the dye-supplying layer, wherein the dye releasing ability of the dye-supplying layer is larger than that of the dye-transferring layer.
In multiple sublimation thermal transfer recording, when a recording material including an overlaid ink layer in which at least the bottom ink layer includes two or more sublimable dyes is used, problems occur in which a color tone of the initially recorded image is different from that of the recorded image after the recording material is used n-times in the n-time multiple sublimation thermal recording or a color tone in a relatively light half tone image is different from that of a relatively dark half tone image.
Because of these reasons, a need exists for a sublimation thermal transfer recording material which can produce images having good image qualities, particularly produce half tone images having good evenness in color tone by a one-time or a multiple sublimation thermal transfer recording method.